Virtual Private Network (VPN) has emerged as a key technology to transport private data over public network infrastructure. VPN has two major components—tunnel technology and security management.
At least two groups of tunnel technologies are available today to encapsulate private data in a format for transportation over a public network such as the Internet. One group of tunnel technologies provides tunnel services at Layer 2 of the OSI network model. (“OSI” is Open System Interconnection.) Point-to-Point Tunnel Protocol (PPTP), Layer 2 Tunnel Protocol (L2TP), and Layer 2 Forwarding (L2F) belongs to the first group. Another group of tunnel technologies provides tunnel services at Layer 3 of the OSI network model. For example, IPSEC (IP Security Protocol) is one of most popular layer-3 tunnel technologies of the second group of tunnel technologies.
Layer 2-type tunnel technologies make use of the existing PPP features to perform authentication and addressing and are preferred for remote access applications to support remote users as shown in FIG. 1. In FIG. 1, Remote user A 102 is a tunnel client who establishes a tunnel (Tunnel 1 104) to Tunnel Server A 106 after being authenticated by the Authentication Server A 108. (A “tunnel server” is a tunnel terminator that terminates a tunnel and forwards traffic within the tunnel to respective “end systems”.) Remote User B 112 uses an Access Concentrator 114 which acts a tunnel client to establish a tunnel (Tunnel 2 116) between Tunnel Server A 106 and Access Concentrator 114 after Remote User B 112 is authenticated by Authentication Server B 118 (and the credential of Remote User B 112 is passed by Access Concentrator 114 and authenticated by Authentication Server A 108). Another kind of tunnel connectivity is a “between site-to-site tunnel” (Tunnel 3 122 in FIG. 1) which is established between Tunnel Server B 124 and Tunnel Server A 106.
As VPN technology becomes more widespread, more VPN enabled networks are constructed and more applications utilize VPN (for example, for telecommuting, extranets and intranets). As a result, it becomes desirable to have the ability to “scale up” tunnel servers to support a larger number of clients and remote users securely and reliably. While “load balancers” exist conventionally, such load balancers handle a group of tunnel servers but do not keep track of tunnel connections or even understand the tunnel protocols. Furthermore, it is desirable that the VPN's be reliably available by, for example, providing a failover and/or backup mechanism and be able to operate at very high speed. In addition, security (e.g., firewall) is desirably provided to protect data from unauthorized access. As discussed above, many conventional tunnel servers implement IPSEC protocol. However, IPSEC is a very time consuming operation and requires a large amount of processing time from tunnel servers, thus limiting the number of VPN client that can be supported by such tunnel servers.